In order to obtain the superior features of polyethylene naphthalate which is used for manufacturing fibers, films, and the like, which are primarily formed of polyethylene naphthalate, it is necessary that 2,6-naphthalene dicarboxylic acid, which is a monomer component for forming a polymeric material, have a high purity, and accordingly, it is also desirable that 2,6-dimethylnaphtahlene (hereinafter, dimethylnaphthalene will be referred to as DMN regardless of the positions of substituent methyl groups) for forming the monomer component have a high purity. DMN has 10 isomers, and a highly pure 2,6-dimethylnaphthalene (preferably having a purity of 99% or more), which is mixed with substantially none of the other 9 isomers, is preferably used.
As a method for manufacturing the 2,6-DMN described above, there may be mentioned a method of separating 2,6-DMN from an isomer mixture obtained by isomerizing 1,5-DMN which is formed by a reaction between orthoxylene and butadiene; a method comprising methylating naphthalene or methylnaphthalenes, subsequently isomerizing, and separating 2,6-DMN; and a method of separating 2,6-DMN from a tar or an oil fraction. However, the fractions and the products described above are each a mixture containing many types of DMN isomers in addition to 2,6-DMN, and hence, the 2,6-DMN must be separated from the mixture described above. However, since the boiling points of these DMN isomers are very close to each other, it has been difficult to separate a highly pure 2,6-DMN therefrom by distillation which is commonly used for separation/purification of organic compounds.
Accordingly, as a method for separating this 2,6-DMN, a crystallization method or an adsorption method has been proposed, and in addition to these methods mentioned above, for example, a method comprising forming a complex by using a certain organic compound, separating the complex, and decomposing this separated complex, and combinations of the methods described above have also been proposed. A cooling crystallization method is a method exploiting the property of 2,6-DMN having a highest melting point among the 10 types of DMN isomers, and since the cooling crystallization method is simple compared to the methods described above, this method can be used suitably as an industrial separation method. However, since it has been difficult to obtain a 2,6-DMN having a purity of 99% or more only by the cooling crystallization method, a process such as treatment using a solvent is generally used together therewith. For example, in Japanese Unexamined Patent Application Publication Nos. 48-5767 and 48-22449, and Japanese Examined Patent Application Publication No. 50-22553, a method has been disclosed in which after a mixture containing DMN isomers is crystallized by cooling, solid-liquid separation is performed by suction filtration, and the obtained solid component is dissolved in a solvent and is then crystallized by cooling. However, according to the related arts described above, a DMN mixture which primarily includes specific DMN isomers among the 10 types of isomers, such as 2,6-DMN, 1,6-DMN, and 1,5-DMN, which are easily isomerized to each other and are easily separated, is used as a feedstock, and a specific DMN mixture in which the content of 2,7-DMN which is difficult to separate from 2,6-DMN is limited to less than 5 mole percent (approximately equivalent to 5 wt) is used. A mixture containing DMN isomers obtained in a typical manufacturing process generally includes 5 wt % or more of 2,7-DMN, and hence, when a mixture containing DMN isomers which includes 5 wt % or more of 2,7-DMN is used as a feedstock in accordance with the related art disclosed in the publications described above, it has been difficult to obtain a highly pure 2,6-DMN.
In view of the situations described above, the present invention was made, and an object of the present invention is to provide a method for manufacturing a highly pure 2,6-DMN even when a mixture containing DMN isomers which includes 5 wt % or more of 2,7-DMN is used as a feedstock.